Traction Control Apparatus

ABSTRACT

An object of the invention is to provide a traction control apparatus capable of suitably controlling an error, if it occurs, between an estimation of a vehicle speed and an actual vehicle speed. A traction control apparatus according to the invention includes a vehicle speed estimator and a driving-force controller. The traction control apparatus includes a vehicle state determiner that determines whether the vehicle speed of the construction vehicle estimated by the vehicle speed estimator and the driving-force control by the driving-force controller are balanced, and a driving-force control changer that changes a driving-force control by the driving-force controller when the vehicle state determiner determines the vehicle speed and the driving-force control to be unbalanced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/142,038, which application is a U.S. National Stage application ofand claims priority to International Application No. PCT/JP2009/071584filed on Dec. 25, 2009, which application claims priority to JapaneseApplication No. 2009-002776 filed on Jan. 8, 2009. The entire contentsof the above applications are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to a traction control apparatus.

BACKGROUND ART

Typically, vehicles such as automobiles are occasionally installed witha traction control apparatus and the like to prevent drive slip. Therehas been known that, when an acceleration operation, a low-μ-roaddriving or the like causes drive slip, such a traction control apparatusperforms a braking control of a brake and driving control of an engineto generate appropriate traction on wheels, thereby preventing wheelslip.

When the traction control apparatus is installed in a two-wheel-drivecar, a vehicle speed can easily be estimated by detecting rotationspeeds of driven wheels (not driving wheels) by a sensor and the like.

However, since all wheels of all-wheel-drive vehicles such as afour-wheel-car are driving wheels, all the wheels may generate driveslip. Accordingly, it is difficult to accurately estimate a vehiclespeed only by detecting rotation speeds of all the wheels.

For this reason, there has been proposed a technique for estimating avehicle speed of such an all-wheel-drive vehicle installed with arotation speed sensor for wheels and an acceleration sensor, thetechnique including selecting a select wheel to be referred based on arotation speed of each of the wheels by the rotation speed sensor andestimating the vehicle speed based on an output from the accelerationsensor (see, for instance, Patent Literature 1).

CITATION LIST Patent Literature

-   Patent Literature 1: JP-A-2001-82199

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The technique disclosed in the Patent Literature 1 presupposes aconvergence of slip that is determined under the condition that therotation speed of each of the wheels is within a predetermined deviationfrom a target rotation speed. However, a construction vehicle such as adump truck is assumed to drive on an irregular ground. Since roadconditions on the ground constantly change and conditions of the wheels(e.g., currently slipping or slippery) also constantly change, a vehiclespeed cannot be estimated with high accuracy due to such a largedisturbance.

Accordingly, when a vehicle keeps on slipping and accelerationintegration is kept for a long time, acceleration errors are accumulatedover time, so that errors of an estimated vehicle speed are increased.

When the vehicle speed is overestimated than an actual vehicle speed,the vehicle is judged to be skidding less than it is actually skiddingand a braking control amount by a traction control apparatus isdecreased to cause a larger slip.

On the other hand, when the vehicle speed is underestimated than anactual vehicle speed, the vehicle is judged to be skidding more than itis actually skidding and a braking control amount by the tractioncontrol apparatus is increased to hamper a driving at an inherentlyavailable vehicle speed.

In other words, when such an erroneously estimated vehicle speed and adriving-force control are kept balanced, an appropriate driving force isunobtainable.

An object of the invention is to provide a traction control apparatuscapable of suitably controlling an error, if it occurs, between anestimation of a vehicle speed and an actual vehicle speed.

Means for Solving the Problems

A traction control apparatus for an all-wheel-drive construction vehicleaccording to an aspect of the invention includes: a vehicle speedestimator that estimates a vehicle speed, the vehicle speed estimatorcomprising a rotation speed detector that detects a rotation speed ofeach of wheels and a reference wheel-speed calculator that calculates areference wheel-speed based on the rotation speed detected by therotation speed detector; a driving-force controller that performs adriving-force control of the construction vehicle based on the vehiclespeed estimated by the vehicle speed estimator; a vehicle statedeterminer that determines whether or not the vehicle speed of theconstruction vehicle estimated by the vehicle speed estimator and thedriving-force control by the driving-force controller are balanced; anda driving-force control changer that changes a driving-force control bythe driving-force controller when the vehicle state determinerdetermines that the vehicle speed and the driving-force control areunbalanced.

According to the above aspect of the invention, when the vehicle speedestimated by the vehicle speed estimator and the driving-force controlby the driving-force controller are determined to be unbalanced, thedriving-force control changer changes the driving-force control, so thaterrors in the estimated vehicle speed can be kept from accumulating andan appropriate control by the traction control apparatus can berecovered.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the driving-force controller includes abraking mechanism controller that controls a braking mechanism of theconstruction vehicle, in which the braking mechanism controllercalculates a slip ratio of each of the wheels based on the rotationspeed detected by the rotation speed detector and controls the brakingmechanism so that the calculated slip ratio converges to a predeterminedtarget value, and the vehicle state determiner determines the vehiclespeed and the driving-force control to be unbalanced when the slip ratiocalculated by the braking mechanism controller exceeds the target valueand is kept at a predetermined threshold or more for a predeterminedtime or more.

With this arrangement, during control by the traction control apparatus,the braking mechanism controller controls the braking mechanism so thatthe slip ratio converges to the predetermined target value. Accordingly,when the slip ratio is kept at the predetermined threshold or more forthe predetermined time or more, it is determined that the vehicle speedis erroneously estimated and the braking mechanism control is balancedwith the erroneously estimated vehicle speed. In such a case, changingthe driving-force control by the driving-force control changer can avoidsuch an unbalance and allows the traction control apparatus to recoverto an appropriate control.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the vehicle state determiner determinesthe vehicle speed and the driving-force control to be unbalanced when abraking control amount of the braking mechanism by the braking mechanismcontroller is kept at a predetermined threshold or more for apredetermined time or more.

With this arrangement, during control by the traction control apparatus,the braking control amount of the braking mechanism kept at thepredetermined threshold or more and not recovered even after thepredetermined time or more is considered to be caused by the erroneousestimation of the vehicle speed and unbalance between the driving-forcecontrol the erroneously estimated vehicle speed. Accordingly, changingthe driving-force control by the driving-force control changer can avoidsuch an unbalance and allows the traction control apparatus to recoverto an appropriate control.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the vehicle state determiner determinesthe vehicle speed and the driving-force control to be unbalanced whenthe slip ratios of right and left ones of the wheels calculated by thebraking mechanism controller are both at a predetermined threshold orless, and the braking control amount of the braking mechanism by thebraking mechanism controller is kept at the predetermined threshold ormore for the predetermined time or more.

With this arrangement, the slip ratio of the predetermined threshold orless and the braking control amount kept at the predetermined thresholdor more for the predetermined time are considered to be caused by theerroneously estimated vehicle speed. Accordingly, changing thedriving-force control by the driving-force control changer can avoidsuch an unbalance and allows the traction control apparatus to recoverto an appropriate control.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the construction vehicle is thearticulate-type dump truck, the articulate angle detector that detectsthe articulate angle of the construction vehicle is provided, and thevehicle state determiner includes a first threshold changing sectionthat changes at least one of the threshold of the slip ratio, thethreshold of the braking control amount and an elapsed time fordetermination in accordance with the detected articulate angle.

With this arrangement, since the vehicle state determiner includes thefirst threshold changing section, the thresholds and the elapsed timefor determination are changed in accordance with a steering angle of theconstruction vehicle detected as the articulate angle, so that thevehicle state determiner can determine whether the vehicle state isbalanced in accordance with a driving condition.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the construction vehicle is thearticulate-type dump truck, the articulate angle detector that detectsthe articulate angle of the construction vehicle is provided, and thevehicle state determiner includes a second threshold changing sectionthat changes at least one of the threshold of the slip ratio, thethreshold of the braking control amount and an elapsed time fordetermination in accordance with the detected articulate angle.

With this arrangement, since the vehicle state determiner includes thesecond threshold changing section, the thresholds and the elapsed timefor determination are changed in accordance with a steering operationalspeed of the construction vehicle detected as the change amount of thearticulate angle per unit time, so that the vehicle state determiner canalso determine whether the vehicle state is balanced in accordance witha driving condition.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the vehicle state determiner determinesthe vehicle speed and the driving-force control to be unbalanced when atleast one of conditions is satisfied, the conditions including: that thebraking control amount of the right and left ones of the wheels by thebraking mechanism controller is the predetermined threshold or more;that a total value of the braking control amount of the right and leftones of the wheels is a predetermined threshold value or more, that atotal value of the braking control amounts of all the wheels is apredetermined threshold or more; and that a total value of a lager oneof the braking control amounts of the right and left front ones of thewheels disposed in a front travel direction and a lager one of thebraking control amounts of the right and left ones of the wheelsdisposed backward from the front wheels and braked by the brakingmechanism is a predetermined value.

With this arrangement, since the braking control amount by the brakingmechanism controller is determined to be excessively large under theabove conditions, changing the driving-force control by thedriving-force control changer can avoid such an unbalance and allow thetraction control apparatus to recover to an appropriate control.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the driving-force control changerchanges a driving-force control by setting the vehicle speed estimatedby the vehicle speed estimator at the reference wheel speed calculatedby the reference wheel-speed calculator.

With this arrangement, when the vehicle state determiner determines thevehicle state to be unbalanced, the driving-force control changer setsthe estimated vehicle speed at the reference wheel speed calculated bythe reference wheel-speed calculator to increase the estimated vehiclespeed, so that the calculated slip ratio is decreased to lessen thebraking control amount by the braking mechanism controller.

In the traction control apparatus according to the aspect of theinvention, it is preferable that the driving-force control changerchanges a driving-force control by releasing the control by thedriving-force controller.

With this arrangement, when the vehicle state determiner determines thevehicle state to be unbalanced, the driving-force control changerreleases the control by the driving-force controller, so that aninappropriate control by the traction control apparatus can be avoided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a structure of a construction vehicleaccording to an exemplary embodiment of the invention.

FIG. 2 is a hydraulic circuit diagram of the construction vehicleaccording to the exemplary embodiment.

FIG. 3 is a functional block diagram of a TCS controller according tothe exemplary embodiment.

FIG. 4 is a functional block diagram of a vehicle speed estimatoraccording to the exemplary embodiment.

FIG. 5 is a functional block diagram of a vehicle speed estimation unitaccording to the exemplary embodiment.

FIG. 6 is a flowchart for illustrating an operation in the exemplaryembodiment.

FIG. 7 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 8 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 9 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 10 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 11 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 12 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 13 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 14 is a flowchart for illustrating an operation of a vehicle statedetermination processing in the exemplary embodiment.

FIG. 15 is a flowchart for illustrating steps of an integrationprocessing of an acceleration and deceleration component in theexemplary embodiment.

FIG. 16 is a flowchart for illustrating a vehicle speed estimationprocessing by the vehicle speed estimation unit in the exemplaryembodiment.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT(S)

Exemplary embodiment(s) of the invention will be described below withreference to the attached drawings.

1. Structure of Dump Truck 1

FIG. 1 shows a dump truck 1 according to an exemplary embodiment of theinvention. The dump truck 1 is an articulated truck that includesseparate front and rear vehicle body frames. A vehicle body of the dumptruck 1 includes an engine 1A, a transmission 1B, differentialmechanisms 1C to 1F and a differential adjusting mechanism 1CA. Theoutput of the engine 1A is controlled by an engine controller 2, and istransmitted to the transmission 1B. The transmission 1B includes atorque converter (not shown). A transmission controller 3 performsgear-shift control on the transmission 1B.

A rotary driving force transmitted from the engine 1A to thetransmission 1B rotates all wheels 4 via the differential mechanisms 1Cto 1F and is transmitted to a road surface.

In this exemplary embodiment, the differential mechanism 1C is providedwith the differential adjusting mechanism 1CA, so that a differential ofthe differential mechanism 1C can be restrained by the differentialadjusting mechanism 1CA. The differential mechanisms 1D and 1E arearranged to accept only the differentials of the right and left wheels.Thus, the differential mechanism 1E is in a so-called direct connectionin which the differentials of the front and rear wheels are notacceptable.

The wheels 4 in the vehicle body are provided with front brakes 41 andcenter brakes 42. The front brakes 41 and the center brakes 42 arehydraulically connected to a brake hydraulic circuit 5 and a TCS controlhydraulic circuit 6 (see FIG. 2).

A braking mechanism includes the front brakes 41, the center brakes 42,the brake hydraulic circuit 5 and the TCS control hydraulic circuit 6(see FIG. 2).

The wheels 4 are respectively provided with rotation speed sensors(i.e., rotation speed detectors) 43FL, 43FR, 43CL and 43CR (which aredescribed later in detail) for detecting the rotation speeds of thewheels 4. A rotation speed signal detected by each of the rotation speedsensors 43FL, 43FR, 43CL and 43CR is output to a TCS controller 7 as anelectrical signal.

The TCS controller 7 includes: an articulate angle sensor 7A fordetecting an articulate angle (bending angle) of the dump truck 1; andan acceleration sensor (acceleration detector) 7D for detectingacceleration applied in a front and rear direction of the dump truck 1.The articulate angle detected by the articulate angle sensor 7A and theacceleration detected by the acceleration sensor 7D are output to theTCS controller 7 as electrical signals.

A TCS system switch 7B for cancelling TCS control is electricallyconnected to the TCS controller 7.

The TCS controller 7 controls the brake torques of the front brakes 41and the center brakes 42 via the hydraulic circuits 5 and 6 and performsan inter-axle differential control for adjusting the differentialrestraining force of the differential adjusting mechanism 1CA. The TCScontroller 7 also functions as a controller for retarder control. TheTCS controller 7 performs the retarder control in accordance with anoperation signal from a retarder control lever 7C used for setting aretarder speed.

2. Structure of Brake Hydraulic Circuit 5

FIG. 2 shows the brake hydraulic circuit 5 of the dump truck 1. In thisexemplary embodiment, the front brakes 41 and the center brakes 42include multi-disc brakes 411 and 421 and slack adjusters 412 and 422,respectively. The slack adjusters 412 and 422 are hydraulicallyconnected to the brake hydraulic circuit 5 and the TCS control hydrauliccircuit 6.

All the front brakes 41 and the center brakes 42 are hydraulicallycontrolled, so that when pressure oil is discharged from the brakehydraulic circuit 5, the discharged pressure oil is supplied to relatedportions of the front brakes 41 and the center brakes 42 via the TCScontrol hydraulic circuit 6, thereby hydraulically driving the relatedportions. The slack adjusters 412 and 422 are devices capable ofautomatically adjusting gaps resulting from abrasion of the front brakes41 and the center brakes 42.

The brake hydraulic circuit 5 includes a hydraulic supply system 51, afoot brake valve 52 and a parking brake valve 53.

The hydraulic supply system 51 includes a plurality of hydraulicaccumulators 511, 512 and 513 as hydraulic sources, a hydraulic pump 514and a reservoir 515. Pressure oil is supplied from the hydraulicaccumulators 511, 512 and 513 to the front brakes 41 and the centerbrakes 42 via the TCS control hydraulic circuit 6, thereby braking thewheels 4.

Each of the hydraulic accumulators 511, 512 and 513 receives thepressure oil in the reservoir 515, the pressure of which is boosted withthe assistance of the hydraulic pump 514 driven by the engine 1A(driving source), to accumulate a predetermined pressure. When thepredetermined pressure is obtained, an unload device 516 disposedbetween the hydraulic pump 514 and the hydraulic accumulator 513 unloadsthe pressure oil from the hydraulic pump 514.

The foot brake valve 52 includes a front brake valve 521 and a centerbrake valve 522. When a brake pedal 523 is operated, the front brakevalve 521 and the center brake valve 522 respectively supply thepressure oil of the hydraulic accumulators 511 and 512 to the frontbrakes 41 and the center brakes 42 for braking.

Specifically, when the brake pedal 523 is operated, the position of thespool of the front brake valve 521 is shifted and the pressure oil ofthe hydraulic accumulator 511 is discharged from the front brake valve521. The pressure oil is supplied to the front brakes 41 via a fronthydraulic circuit 61 in the TCS control hydraulic circuit 6 to effectthe braking of the front brakes 41.

More specifically, the pressure oil discharged from the front brakevalve 521 acts on the right and left front brakes 41 with asubstantially equal pressure via shuttle valves 614 and 615, therebyequally performing the braking on the right and left sides.

The pressure oil discharged from the center brake valve 522 acts on theright and left center brakes 42 with a substantially equal pressure viashuttle valves 624 and 625, thereby equally performing the braking onthe right and left sides.

Simultaneously, the position of the spool of the center brake valve 522is shifted, so that the pressure oil of the hydraulic accumulator 512 isdischarged from the center brake valve 522. The pressure oil is suppliedto the center brake 42 via a center hydraulic circuit 62 to effect thebraking of the center brakes 42.

The parking brake valve 53 is a valve for controlling a parking brake54. The parking brake valve 53 includes a solenoid 531 and a spring 532.When a parking switch disposed in an operation room (not shown) isswitched to a parking position, and thus, the position of the parkingbrake valve 53 is shifted with the assistance of the solenoid 531, theparking brake valve 53 supplies pressure oil in the hydraulicaccumulator 513 to a cylinder chamber 541 of the parking brake 54,thereby increasing a parking brake pressure. As a result, when thevehicle is parted, the braking condition is maintained.

Although shown in the upper left in FIG. 2, practically, the parkingbrake 54 is provided in parallel with the front brakes 41 or the centerbrakes 42, or is provided to a brake attached to a drive shaft thattransmits a driving force.

When the vehicle travels, a parking switch (not shown) is switched to atravel position, and thus, the position of the parking brake valve 53 isshifted to a position where the pressure oil from the hydraulicaccumulator 513 is blocked, and directs the pressure oil in a cylinderchamber 541 of the parking brake 54 back to the reservoir 515 of thehydraulic supply system 51, thereby reducing a parking brake pressure tozero. As a result, when the vehicle travels, the vehicle is movable.

3. Structure of TCS Control Hydraulic Circuit 6

As shown in FIG. 2, the TCS control hydraulic circuit 6 is disposed inthe middle of a hydraulic circuit extending from the brake hydrauliccircuit 5 to the front brakes 41 and the center brakes 42. The TCScontrol hydraulic circuit 6 includes a front hydraulic circuit 61 and acenter hydraulic circuit 62.

The front hydraulic circuit 61 is a hydraulic circuit configured toperform the TCS control on the front brakes 41. The front hydrauliccircuit 61 includes a front TCS switching valve 611, two solenoidproportional control valves 612 and 613, the two shuttle valves 614 and615 and pressure sensors 616 and 617.

The front TCS switching valve 611 is capable of switching whether or notto perform the TCS brake control on the front brakes 41 in response toan electric signal output from the TCS controller 7 to a solenoid 611Aof the switching valve 611.

The solenoid proportional control valves 612 and 613 are control valvesthat are respectively disposed on pipe lines branched in the middle of apipe line having an end connected to the output side of the front TCSswitching valve 611. The solenoid proportional control valves 612 and613 are configured to control the brake pressure of the front brakes 41during the TCS control. The solenoid proportional control valve 612 is avalve configured to control pressure oil supply to the left one of thefront brakes 41. The solenoid proportional control valve 613 is a valveconfigured to control pressure oil supply to the right one of the frontbrakes 41.

The opening degrees of the solenoid proportional control valves 612 and613 are respectively adjusted by the solenoids 612A and 613A. Afterbeing depressurized and discharged, the hydraulic oil is partly directedback to the reservoir 515 of the above hydraulic supply system 51.

The shuttle valves 614 and 615 are disposed on the output sides of thesolenoid proportional control valves 612 and 613, respectively. Theshuttle valves 614 and 615 have, on one sides thereof, inputs beingconnected to outputs from the solenoid proportional control valve 612and 613, and, on the other sides thereof, inputs being connected to eachother via a pipe that communicates the inputs of the shuttle valves 614and 615 to each other. In the middle of this pipe, an output pipe forthe front brake valve 521 is connected.

The pressure sensors 616 and 617 are respectively disposed in themiddles of pipes extending between the shuttle valves 614 and 615 andthe solenoid proportional control valves 612 and 613. The pressuresensors 616 and 617 are configured to detect the brake pressure of thefront brakes 41 and to output the detected signals to the TCS controller7 as electric signals. The pressure sensors 616 and 617 may be disposedin the middles of pipes extending between the shuttle valves 614, 615,624 and 625 and the slack adjusters 412 and 422.

The center hydraulic circuit 62 is a hydraulic circuit configured toperform the TCS control on the center brakes 42. The center hydrauliccircuit 62 includes a center TCS switching valve 621, two solenoidproportional control valves 622 and 623, the two shuttle valves 624 and625, and pressure sensors 626 and 627 in the same manner as the fronthydraulic circuit 61.

Likewise, the solenoid proportional control valves 622 and 623 arerespectively provided with solenoids 622A and 623A. The opening degreeof each of the solenoid proportional control valves 622 and 623 isadjusted in accordance with an electric signal output from the TCScontroller 7.

The center TCS switching valve 621 is also provided with a solenoid621A. The center TCS switching valve 621 switches whether or not toperform TCS on the center brakes 42 in accordance with an electricsignal output from the TCS controller 7.

The TCS control hydraulic circuit 6 enables a TCS function through theshifting of the positions of the valves of the above front hydrauliccircuit 61 and center hydraulic circuit 62.

When the spool of the front TCS switching valve 611 is set at an upperposition and the spool of the center TCS switching valve 621 is set atan upper position in FIG. 2, the TCS function is disabled.

In contrast, when the spool of the front TCS switching valve 611 is setat a lower position and the spool of the center TCS switching valve 621is set at a lower position in FIG. 2, the TCS function is enabled.

In this case, in the front hydraulic circuit 61, the pressure oildischarged from the front TCS switching valve 611 is supplied to thesolenoid proportional control valves 612 and 613. The opening degrees ofthe solenoid proportional control valves 612 and 613 are adjusted inaccordance with an electric signal from the TCS controller 7. Thepressure oil discharged from the solenoid proportional control valves612 and 613 is supplied to the front brakes 41 via the shuttle valves614 and 615.

In the center hydraulic circuit 62, the pressure oil discharged from thecenter TCS switching valve 621 is supplied to the solenoid proportionalcontrol valves 622 and 623. The pressure oil discharged from thesolenoid proportional control valves 622 and 623 is supplied to thecenter brakes 42 via the shuttle valves 624 and 625.

At this time, the TCS controller 7 monitors the rotation speeds of thewheels 4 detected by the rotation speed sensors 43FL, 43FR, 43CL and43CR, and outputs electric signals to the solenoids 612A, 613A, 622A and623A in accordance with the slip ratios of the wheels 4 (which will bedescribed later in detail). As a result, the opening degrees of thesolenoid proportional control valves 612, 613, 622 and 623 are adjusted,thereby adjusting the braking forces of the front brakes 41 and centerbrakes 42. In this manner, while adjusting the driving force of each ofthe wheels 4 at an optimum value, the TCS controller 7 performs controlfor ensuring course-traceability when the vehicle is turned.

When the brake pedal 523 is operated, on the front side, the pressureoil discharged from the front brake valve 521 is supplied to the frontbrakes 41 via the shuttle valves 614 and 615, so that each of the frontbrakes 41 functions as a normal brake that increases the braking forcethereof in accordance with the pressed amount of the brake pedal 523. Onthe rear side, the pressure oil discharged from the center brake valve522 is supplied to the center brakes 42 via the shuttle valves 624 and625, and each of the center brakes 42 likewise functions as a normalbrake.

The solenoid proportional control valves 612, 613, 622 and 623 are alsoused as control valves for retarder control. The opening degree of eachof the solenoid proportional control valves 612, 613, 622 and 623 isadjusted in accordance with a retarder command signal from the TCScontroller 7.

4. Structure of TCS Controller 7

FIG. 3 shows the structure of the TCS controller 7 that performs theabove TCS control.

The TCS controller 7 includes a memory 71 as a storage and a processor72.

The memory 71 stores not only a program executable on the processor 72but also a map for TCS sliding mode control and the like, which arereadable upon a request from the processor 72.

The rotation speed sensors 43FL, 43FR, 43CL and 43CR, the articulateangle sensor 7A, the TCS system switch 7B, the retarder control lever 7Cand the pressure sensors 616, 617, 626 and 627 are connected to theinput side of the processor 72.

The rotation speed sensors 43FL, 43FR, 43CL and 43CR and theacceleration sensor 7D are connected to the processor 72 via LPFs (LowPass Filter) 73 and 74, so that rotation speed signals output from therotation speed sensors 43FL, 43FR, 43CL and 43CR, and an accelerationsignal output from the acceleration sensor 7D, from which ahigh-frequency component such as disturbance has been eliminated, areinput to the processor 72 as rotation speeds ωfl, ωfr, ωcl and ωcr andas an acceleration acting in a travel direction of the dump truck 1.

In contrast, the solenoids 611A and 621A of the TCS switching valves 611and 621 and the solenoids 612A, 613A, 622A and 623A of the solenoidproportional control valves 612, 613, 622 and 623 of the TCS controlhydraulic circuit 6 are electrically connected to the output side of theprocessor 72.

The processor 72 is also electrically connected to the engine controller2 and the transmission controller 3 so that information is exchangeabletherebetween. Thus, the processor 72 can acquire various kinds ofinformation required for the TCS control and the inter-axle differentialcontrol from the engine controller 2 and the transmission controller 3,such as an output torque value of the engine from the engine controller2, and speed stage information and lock-up information from thetransmission controller 3.

The processor 72 includes a vehicle speed estimator 80, acontrol-permission determiner 81, a control-start determiner 82, acontrol-termination determiner 83, a braking mechanism controller 84, adifferential adjusting mechanism controller 85 and a retarder controller86.

The braking mechanism controller 84 and the differential adjustingmechanism controller 85 are components of a driving-force controlleraccording to the invention.

The control-permission determiner 81 determines whether or not to permitthe TCS control. Specifically, the control-permission determiner 81determines whether or not to permit the TCS control based on anoperation condition of the TCS system switch 7B, an operation conditionof the brake pedal 523, the speed stage information of the transmission1B, a control condition of the retarder control, and an operationcondition of an accelerator pedal (not shown).

The control-start determiner 82 is a section for determining whether ornot start conditions for the TCS brake control have been fulfilled. Thedetermination of starting conditions is based on the rotation speedsignal of each of the wheels 4 detected by the rotation speed sensors43FL, 43FR, 43CL and 43CR. Specifically, the control-start determiner 82determines to start at least one of the TCS control and the inter-axledifferential control when a rotation speed difference of the right andleft wheels and a rotation speed difference of the front and rear wheelsreach or exceed a predetermined threshold stored in the memory 71.

The control-termination determiner 83 is a section for determiningwhether or not to terminate the TCS control and the inter-axledifferential control. In this exemplary embodiment, thecontrol-termination determiner 83 determines whether or not to terminatethe brake control on the front wheels 4, the brake control on the centerwheels 4, and the inter-axle differential control with reference to acontrol deviation of each of the wheels 4 obtained by the brakingmechanism controller 84.

The braking mechanism controller 84 generates and outputs a controlcommand for the TCS. For generating the control command, an actual slipratio λ of each of the wheels 4 is calculated by the following equation(1) based on a vehicle speed V of the dump truck 1 estimated by thelater-described vehicle speed estimator 80, a radius r of the wheels 4,and the rotation speeds ωfl, ωfr, ωcl and ωcr of the respective wheels4.

λ=(rλ−V)/rω  (1)

Next, the braking mechanism controller 84 calculates the target slipratio η for each of the wheels 4 by applying the reference target slipratio ηs stored in the memory 71 and the modifying target slip ratio ηaset in accordance with the articulate angle detected by the articulateangle sensor 7A in the following equation (2).

η=ηs+ηa  (2)

The braking mechanism controller 84 calculates the control deviation Sby applying the calculated actual slip ratio λ and the target slip ratioη in the following equation (3).

S=λ−η  (3)

Next, the braking mechanism controller 84 estimates a force transmittedfrom the wheels 4 to the road surface (i.e., traction force) based onthe output torque of the engine sent from the engine controller 2, speedstage information sent from the transmission controller 3, and thespecification data of the dump truck 1 having been stored in the memory71.

The braking mechanism controller 84 applies a control law of slidingmode control to the vehicle model of the dump truck 1 based on thecalculated control deviations S and the estimated traction force togenerate and output a control command to solenoids 611A, 612A, 613A,621A, 622A and 623A of the TCS control hydraulic circuit 6, therebycontrolling the braking forces of the wheels 4.

The differential adjusting mechanism controller 85 generates a controlcommand for controlling the differential restraining force of thedifferential mechanism 1C and outputs the generated control command tothe differential adjusting mechanism 1CA. Specifically, when theinter-axle differential control is determined to be performed by thecontrol-start determiner 82, the differential adjusting mechanismcontroller 85 generates a control command for restraining thedifferential of the differential mechanism 1C and outputs the controlcommand to the differential adjusting mechanism 1CA.

The retarder controller 86 enables a constant-speed travel control ofthe dump truck 1 based on information such as a payload of the dumptruck 1 and slope conditions detected by the acceleration sensor 7D.When the retarder control lever 7C is ON, the retarder controller 86generates and outputs a control command to the solenoids 611A, 612A,613A, 621A, 622A and 623A and controls the braking of the front brakes41 and the center brakes 42, thereby performing a constant-speed travelcontrol.

5. Structure of Vehicle Speed Estimator 80

FIG. 4 shows a detailed structure of the vehicle speed estimator 80. Thevehicle speed estimator 80 includes a reference wheel-speed calculator801, a reference speed calculator 802, and a vehicle speed estimationunit 805.

The reference wheel-speed calculator 801 selects a minimum rotationspeed (ωmin) among the rotation speeds (ωfl, ωfr, ωcl and ωcr) of thewheels 4 respectively detected by the rotation speed sensors 43FL, 43FR,43CL and 43CR, removes high frequency components from a signal of theselected rotation speed ωmin by LPF73, and then calculates a referencewheel-speed Vre1 by the following equation (4) with the radius r of thewheels 4.

Vrel=r×ωmin  (4)

The rotation speed ω among those of the wheels 4 having the minimumrotation speed ω is selected because the wheel 4 having the minimumrotation speed ω is skidding the least among all the wheels 4 of thedump truck 1.

The reference speed calculator 802 calculates a reference speed Vre2from an acceleration filter value input through LPF74. Specifically, thereference speed calculator 802 calculates the acceleration filter valueinput during travel of the dump truck 1 as an acceleration anddeceleration component, add a value of integral of the acceleration anddeceleration component to the previously estimated vehicle speed V inaccordance with the travel condition of the dump truck 1, and sets thereference speed Vre2 which is a candidate of another estimated vehiclespeed V. Under the conditions that the acceleration and decelerationcomponent is less than zero and the transmission 1B is released from alockup, the reference speed calculator 802 does not execute theintegration processing and sets the previously estimated vehicle speed Vas the reference speed Vre2 so as to avoid further speed-reduction inaccordance with increasing brake commands from the TCS control (whichwill be described later in detail).

The vehicle speed estimation unit 805 estimates the vehicle speed V tobe finally used in the equitation (1) in the TCS control of the brakingmechanism controller 84 based on the reference wheel speed Vre1calculated by the reference wheel-speed calculator 801 and the referencespeed Vre2 calculated by the reference speed calculator 802.

As shown in FIG. 5, the vehicle speed estimation unit 805 includes avehicle speed setting unit 814, a vehicle state determiner 815 and adriving-force control changer 816.

The vehicle speed setting unit 814 finally sets the vehicle speed Vbased on the reference speed Vre2 and the reference wheel speed Vre1 tobe input.

The vehicle speed setting unit 814 determining whether the referencespeed Vre2 calculated by the reference speed calculator 802 isexcessively high or low. When determining the reference speed Vre2 iserroneously calculated, the vehicle speed setting unit 814 estimates thevehicle speed V as the reference wheel speed Vre1. The vehicle speed Vis estimated in accordance with the travel conditions of the dump truck1 (which will be described later in detail), as shown in the followingTable 1.

TABLE 1 JUDGING CONDITIONS PROCESSING CONTENTS accelerator off or Vre1 <0.3 m/s V = Vre1 Vre2 < Vre1 × 0.5 V = Vre1 × 0.5 Vre2 > Vre1 V = Vre1less than 0.1 of control deviations of all The previous estimated wheelwheels and speed is maintained. less than −0.1 m/s² of acceleration anddeceleration component

The vehicle state determiner 815 determines whether the previouslyestimated vehicle speed V calculated by the vehicle speed setting unit814 and the braking control by the braking mechanism controller 84 arebalanced, and includes a first threshold changing section 815A and asecond threshold changing section 815B.

The vehicle state determiner 815 determines whether or not the vehiclespeed V estimated by the vehicle speed setting unit 814 and the brakingcontrol by the braking mechanism controller 84 are balanced, based onthe determinations of whether or not the control deviations S calculatedby the above equations (1) to (3) fall within a predetermined range andwhether or not the braking control amount output from the brakingmechanism controller 84 is a predetermined threshold or more (which willbe described later in detail).

When the vehicle state determiner 815 determines a threshold forbalance, the first threshold changing section 815A changes a thresholdof the control deviations S, a threshold of the braking control amountand an elapsed time setting for determination, depending on whether ornot the articulate angle output from the articulate angle sensor 7Aexceeds, for instance, 20 degrees.

When the vehicle state determiner 815 determines a threshold forbalance, the second threshold changing section 815B changes the range ofthe control deviations S, the threshold of the braking control amountand the elapsed time setting for determination, depending on whether ornot a change amount per unit time of an articulate angle signal outputfrom the articulate angle sensor 7A exceeds, for instance, 10 degreesper second.

The driving-force control changer 816 changes the braking controlperformed by the braking mechanism controller 84 when the vehicle statedeterminer 815 determines that the estimated vehicle speed V and thebraking control amount are not balanced and kept in an inappropriatestate. Specifically, when the estimated vehicle speed V and the brakingcontrol amount are unbalanced, the driving-force control changer 816outputs a release signal for releasing TCS control by a TCS controller 7to the control-termination determiner 83. The driving-force controlchanger 816 also outputs a signal of V=Vre1 to the braking mechanismcontroller 84 or the vehicle speed setting unit 814.

6. Operation and Effects of Vehicle Speed Estimator 80

Next, an operation of the above-described vehicle speed estimator 80will be described with reference to FIGS. 6 to 16.

As shown in FIG. 6, the vehicle speed estimator 80 performs an inputprocessing S1 for inputting various data, a reference wheel-speedcalculation processing S2, an acceleration signal filter processing S3,a vehicle state determination processing S4, an acceleration anddeceleration integration processing S6 and a reference speed adjustmentprocessing S8, thereby estimating the vehicle speed of the dump truck 1.The processings S1 to S 8 will be respectively described in detail. Theprocessings S1 to S 8 are repeated in a predetermined cycle.

(6-1) Input Processing S1

In order to operate the vehicle speed estimator 80 to estimate a vehiclespeed, various state data of the dump truck 1 are input to the vehiclespeed estimator 80. Specifically, the rotation speeds from the rotationspeed sensors 43FL, 43FR, 43CL and 43CR, flag information showingwhether or not the TCS control is under operation, flag informationshowing whether or not a lockup is switched, flag information showingwhether an acceleration operation is ON or OFF, and flag informationshowing whether or not rotation deviation of the right and left wheelsoccurs are input to the vehicle speed estimator 80.

(6-2) Reference Wheel-Speed Calculation Processing S2

The reference wheel-speed calculation processing S2 is performed by thereference wheel-speed calculator 801. Specifically, the referencewheel-speed calculator 801 initially selects a maximum rotation speedωmax and the minimum rotation speed ωmin from the rotation speeds ωfl,ωfr, ωcl and ωcr of the wheels 4 respectively input from the rotationsensors 43FL, 43FR, 43CL and 43CR, and calculates the maximum referencewheel speed and the minimum reference wheel speed by the equation (4).

Next, the reference wheel-speed calculator 801 calculates variation inthe reference wheel speeds of the wheels 4 by a difference between themaximum reference wheel speed and the minimum reference wheel speed.

Finally, the reference wheel-speed calculator 801 selects the minimumreference wheel speed as the reference wheel speed Vre1.

(6-3) Acceleration Signal Filter Processing S3

In the acceleration signal filter processing S3, the acceleration signaloutput from the acceleration sensor 7D is filtered by LPF74 to removenoises, vehicle vibration components and the like and the thus obtainedacceleration filter value by the filter processing is output to thevehicle speed estimator 80.

(6-4) Vehicle State Determination Processing S4

The vehicle state determination processing S4 is performed by thevehicle state determiner 815 of the vehicle speed estimator 805. Asshown in FIG. 7, the vehicle state determination processing S4 includesa stop-and-backward-movement determination processing S41 of the dumptruck 1, a first vehicle-speed error-estimation determination processingS42, a second vehicle-speed error-estimation determination processingS43, a lockup determination processing S44, a control cancellationdetermination processing S45, a gear-shift determination processing S46and a front-rear rotation speed difference determination processing S47.

In the stop-and-backward-movement determination processing S41, when themaximum reference wheel speed is 0 or less and the accelerationoperation is OFF, it is determined that the dump truck 1 is stopped.Since a forward direction of the dump truck 1 is set as positive in thevehicle speed estimation in this exemplary embodiment, when a gear-shiftof the dump truck 1 is set at R1 or R2, the dump truck 1 is determinedas going backward and the acceleration filter value is reversed tonegative.

As shown in the flowchart of FIG. 8, in Steps S420 to S424 of the firstvehicle-speed error-estimation determination processing S42, values ofcounters 1 to 5 are incremented or cleared in accordance with the valuesof the control deviations S calculated by the braking mechanismcontroller 84, thereby determining whether or not the values of thecontrol deviations S elapsed in a predetermined range for apredetermined time.

Specifically, in Step S420, it is determined whether or not the controldeviation S of each of the wheels 4 is in a range of 0.05 to 0.2. Asshown in the flowchart of FIG. 9, any one of the wheels 4 (front-right,front-left, rear-right, rear-left) is selected (Step T1) and it isdetermined whether or not a value of a control deviation S of theselected wheel 4 is in the range of the control deviation S set in eachof the Steps S420 to S424 (Step T2).

For instance, when the value of the control deviation S of thefront-left wheel 4 is in a range of S=0.05 to 0.2, the counter 1 isincremented (Step T3). When not in the range, the counter 1 is cleared(Step T4). These steps are repeated until the determination of the valueof the control deviation S for each of all the wheels 4 is terminated(Step T5).

Subsequently, similarly, in Step S421, when the value of the controldeviation S for each of the wheels 4 is in a range of 0.125 to 0.275,the counter 2 is incremented. When not in the range, the counter 2 iscleared.

In Step S422, the counter 3 is incremented or cleared depending onwhether or not the value of the control deviation S for each of thewheels 4 is in a range of 0.2 to 0.35. In Step S423, the counter 4 isincremented or cleared depending on whether or not the value of thecontrol deviation S for each of the wheels 4 is in a range of 0.275 to0.425. In Step S424, the counter 5 is incremented or cleared dependingon whether or not the value of the control deviation S for each of thewheels 4 is in a range of 0.35 to 0.5.

After completing the above determination processing of the controldeviations S, the vehicle state determiner 815 determines whether or notany one of the values of the counters 1 to 5 of each of the wheels 4 islarger than, for instance, 400 (4 seconds) (Step S425). When the valueis larger, it is determined that the slip ratio is larger than theoriginal target value and is kept for a predetermined time (4 seconds)or more and the vehicle speed V set by the vehicle speed setting unit814 is different from the actual vehicle speed, whereby a firsterror-estimation flag is set to be ON (Step S426). Thus, the processingis terminated.

On the other hand, when it is determined that all of the values of thecounters 1 to 5 of each of the wheels 4 are 400 or less, the vehiclestate determiner 815 determines that the vehicle speed V set by thevehicle speed setting unit 814 is correct, whereby the firsterror-estimation flag is set to be OFF (Step S427). Thus, the processingis terminated.

Referring to FIG. 7, after completing the first vehicle-speederror-estimation determination processing in Step S42, the vehicle statedeterminer 815 performs the second vehicle-speed error-estimationdetermination processing.

In the second vehicle-speed error-estimation determination processingS43, the processings shown in the flowchart of FIG. 10 are performed todetermine whether or not the vehicle speed V set by the vehicle speedsetting unit 814 is erroneous, because the vehicle speed V is highlypossibly erroneous under the conditions of a small control deviation, alarge braking control amount, and a predetermined elapsed time.

First, the vehicle state determiner 815 determines whether or not bothof the braking control amounts of the right and left center wheels 4 area predetermined threshold K1 or more and the values of the controldeviations S of the right and left center wheels 4 are less than 0.1, orwhether or not the articulate angle detected by the articulate anglesensor 7A exceeds 20 degrees (Step S430).

When the above conditions are satisfied, the vehicle state determiner815 increments the center counter (Step S431). When the above conditionsare not satisfied, the vehicle state determiner 815 clears the centercounter (Step S432).

Subsequently, the vehicle state determiner 815 determines whether or notthe articulate angle exceeds 20 degrees (Step S433).

When determining the articulate angle exceeds 20 degrees, the vehiclestate determiner 815 determines whether or not the center counterexceeds 100 (1 second) (Step 434). When the center counter exceeds 100(1 second), since the vehicle is rotating with a large articulate angleand a difference between an inner wheel and an outer wheel occurs due torotation, the vehicle speed V is possibly erroneously estimated.Accordingly, the vehicle state determiner 815 sets the firsterror-estimation flag to be ON (Step S435). When the center counter doesnot exceed 100 (1 second), the vehicle state determiner 815 sets thesecond error-estimation flag to be OFF (Step S436). Thus, the processingis terminated.

When determining the articulate angle is 20 degrees or less, the vehiclestate determiner 815 determines whether or not the center counterexceeds, for instance, 300 (3 second) (Step 437). When the centercounter exceeds 300 (3 second), the vehicle state determiner 815 setsthe second error-estimation flag to be ON (Step S438). When the centercounter does not exceed 300 (3 second), the vehicle state determiner 815sets the second error-estimation flag to be OFF (Step S439). Thus, theprocessing is terminated.

Referring to FIG. 7, after completing the second vehicle-speederror-estimation determination processing S43, the vehicle statedeterminer 815 performs the lockup determination processing S44. Thelockup determination processing S44 is a processing for determining alockup state when a gear-shift command is given to shift to a positionother than the neutral position. Switch flag information is determinedto be ON for a predetermined time after a lockup release command isoutput and to be OFF after the predetermined time.

After the lockup determination processing S44, the vehicle statedeterminer 815 performs the control cancellation determinationprocessing S45 for determining whether or not to cancel the TCS control.

As shown in the flowchart in FIG. 11, in the control cancellationdetermination processing S45, a counter processing S450 of the frontwheels 4, a counter processing S451 of the center wheels 4, a totalbraking-control-amount determination counter processing S452 of thefront and center wheels, and a control-cancellation-flag andunder-control determination-cancellation-flag determination processingS453 are sequentially executed.

In the counter processing S450 of the front wheels 4, Steps T6 to T10shown in the flowchart of FIG. 12 are executed.

First, the vehicle state determiner 815 determines whether or not thebraking control amounts of the right and left front wheels 4 are apredetermined threshold or more and the values of the control deviationsS of the right and left front wheels 4 are less than 0.1, whether or notthe articulate angle exceeds 20 degrees, or whether or not the change inthe articulate angle per unit time exceeds 10 degrees per second (StepT6).

When determining all the conditions are satisfied, the vehicle statedeterminer 815 increments the front counters and sets the value of thefront delay counter at zero (Step T7).

When any one of the conditions is not satisfied, the vehicle statedeterminer 815 increments the front delay counters (Step T8), anddetermines whether or not the front delay counter exceeds 100 (1 second)(Step T9). When the front delay counter does not exceed 100, theprocessing is terminated.

When the front delay counter exceeds 100, the vehicle state determiner815 sets the front counter at zero and sets the front delay counter at101 (Step T10). Thus, the processing is terminated.

In the counter processing S451 of the center wheels 4, the value of thecenter counter and the value of the center delay counter are set in thesame manner as the above. Thus, the processing is terminated.

Next, the vehicle state determiner 815 executes the totalbraking-control-amount determination counter processing S452 of thefront and center wheels, which is specifically the processing shown inthe flowchart of FIG. 13.

The vehicle state determiner 815 acquires the braking control amount ofeach of the wheels 4 output from the braking mechanism controller 84 andcalculates a total braking control amount (Step T11). The total brakingcontrol amount is calculated by adding a lager one of the brakingcontrol amounts of the right and left front wheels 4 and a lager one ofthe braking control amounts of the right and left center wheels 4.

The vehicle state determiner 815 determines whether or not the totalbraking control amount is a predetermined threshold or more (Step T12).When determining the total braking control amount is the predeterminedthreshold or more, the vehicle state determiner 815 increments a totalbraking-control-amount counter (Step T13). When determining the totalbraking control amount is not at the predetermined threshold or more,the vehicle state determiner 815 sets the total braking-control-amountcounter at zero (Step T14) to complete the processing.

Finally, the vehicle state determiner 815 executes thecontrol-cancellation-flag and under-controldetermination-cancellation-flag determination processing S453.Specifically, the vehicle state determiner 815 executes the processingsof FIG. 14.

The vehicle state determiner 815 determines whether or not thearticulate angle exceeds 20 degrees (Step T15). When the articulateangle exceeds 20 degrees, the vehicle state determiner 815 furtherdetermines whether or not the front counters or the center countersexceed 100 (1 second) or whether or not the total braking control amountcounter exceeds 200 (2 seconds) (Step T16). When the front counters orthe center counters exceed 100 (1 second) or the total braking controlamount counter exceeds 200 (2 seconds), the estimation of the vehiclespeed V is possibly erroneous during rotation. Accordingly, in order toavoid excessive braking, the control cancellation flag is set to be ON(Step T17). When the front counters or the center counters do not exceed100 (1 second) or the total braking control amount counter does notexceed 200 (2 seconds), the control cancellation flag is set to be OFF(Step T18).

When the articulate angle is 20 degrees or less, the vehicle statedeterminer 815 further determines whether or not the front counters orthe center counters exceed 300 (3 seconds) or whether or not the totalbraking control amount counter exceeds 200 (2 seconds) (Step T19). Whenthe front counters or the center counters exceed 300 (3 seconds) or thetotal braking control amount counter exceeds 200 (2 seconds), theestimation of the vehicle speed V is possibly erroneous during rotation.Accordingly, in order to avoid excessive braking, the controlcancellation flag is set to be ON (Step T20). When the front counters orthe center counters do not exceed 300 (3 seconds) or the total brakingcontrol amount counter does not exceed 200 (2 seconds), the controlcancellation flag is set to be OFF (Step T21).

Next, the vehicle state determiner 815 performs the gear-shiftdetermination processing S46. The gear-shift determination processingS46 is a processing for determining whether or not the transmission 1Bis shifting gears, i.e., the gear-shift determination processing S46determines a gear-shift state of the transmission 1B based on agear-shift signal of the transmission controller 3.

Finally, the vehicle state determiner 815 performs the front-rearrotation speed difference determination processing S47. The front-rearrotation speed difference determination processing S47 is a processingfor determining whether or not a difference in rotation between a frontoutput shaft and a rear output shaft of the transmission 1B is large.When the difference in rotation between the front output shaft and therear output shaft is a predetermined threshold or more, it is determinedthat there is the deference therebetween.

As described above, the control-termination determiner 83 outputs acommand of whether or not to cancel the TCS control to the brakingmechanism controller 84 based on various flag information set in thevehicle state determination processing S4. The braking mechanismcontroller 84 performs or cancels the TCS control based on the command.

(6-5) Acceleration and Deceleration Component Integration Processing S6

As shown in the flowchart of FIG. 15, first, the reference speedcalculator 802 calculates the acceleration filter value as theacceleration and deceleration component (Step S61).

Next, the reference speed calculator 802 determines whether or not thecalculated acceleration and deceleration component is less than zero andthe lockup switch flag information is ON (Step S62).

When determining that the calculated acceleration and decelerationcomponent is less than zero and the lockup switch flag information is ON(Step S62), the reference speed calculator 802 sets the previouslyestimated vehicle speed V as the reference speed Vre2 (Step S63).

On the other hand, when either of the conditions shown in Step S62 isnot satisfied, the reference speed calculator 802 adds a value ofintegral of the acceleration and deceleration component to thepreviously estimated vehicle speed V (Step S64). The value of integralof the acceleration and deceleration component is calculated bymultiplying the value of the acceleration and deceleration componentthat is calculated by a predetermined sampling cycle by a sampling time.

(6-6) Reference Speed Adjustment Processing S8

The vehicle speed estimation unit 805 executes the reference speedadjustment processing S8, which is specifically the processing shown inthe flowchart of FIG. 16.

First, the vehicle speed estimation unit 805 determines whether theacceleration is OFF or the reference wheel speed Vre1 is less than 0.3msec (Step S81).

When either of the conditions in Step S81 is satisfied, the vehiclespeed estimation unit 805 determines that the vehicle is not skidding.The vehicle speed estimation unit 805 sets the estimated vehicle speed Vat the reference vehicle speed Vre1 and sets an amount provided bysubtracting the reference vehicle speed Vre1 from the reference speedVre2 as a value-of-integral adjusted amount (Step S82).

When the conditions in Step S81 are not satisfied, the vehicle speedestimation unit 805 determines whether the reference speed Vre2 is lessthan a predetermined value (e.g., less than half) of the referencevehicle speed Vre1 (Step S83).

When the condition in Step S83 is satisfied, the vehicle speedestimation unit 805 determines that, when the estimated vehicle speed Vis excessively low, braking may be excessively applied in the TCSbraking control, and sets the estimated vehicle speed V at, forinstance, half of the reference vehicle speed Vre1 and sets an amountprovided by subtracting half of the reference vehicle speed Vre1 fromthe reference speed Vre2 as a value-of-integral adjusted amount (StepS84).

When the condition in Step S83 is not satisfied, the vehicle speedestimation unit 805 determines whether the reference speed Vre2 ishigher than the reference vehicle speed Vre1 (Step S85).

When the condition in Step S85 is satisfied, the vehicle speedestimation unit 805 determines that the estimated vehicle speed V doesnot exceed the reference vehicle speed Vre1. The vehicle speedestimation unit 805 sets the estimated vehicle speed V at Vre1 and setsan amount provided by subtracting the reference vehicle speed Vre1 fromthe reference speed Vre2 as a value-of-integral adjusted amount (StepS86).

When the condition in Step S85 is not satisfied, the vehicle speedestimation unit 805 determines whether the control deviations S of allthe wheels calculated by the braking mechanism controller according tothe equitation (3) are less than 0.1 and the acceleration anddeceleration component is less than −0.1 m/sec² (Step S87).

When the conditions in Step S87 are satisfied, the vehicle speedestimation unit 805 determines that, when reducing a vehicle speed inspite of the control deviations S being in an appropriate range, thecontrol deviations S may be further increased to cause an increase in abrake force by the braking mechanism control to further reduce thevehicle speed. Accordingly, the vehicle speed estimation unit 805 setsthe estimated vehicle speed V at the previously estimated vehicle speedV and sets an amount provided by subtracting the previously estimatedvehicle speed V from the reference speed Vre2 as a value-of-integraladjusted amount (Step S88).

When the conditions in Step S87 are not satisfied, the vehicle speedestimation unit 805 sets the reference speed Vre2 calculated by thereference speed calculator 802 as the estimated vehicle speed V (StepS89).

The invention is not limited to the above-described exemplaryembodiment, but is applicable to the following.

Specifically, the above-described vehicle speed estimator 80 is appliedto an articulate-type dump truck 1, however, an application of theinvention is not limited thereto. More specifically, the invention isapplicable to a rigid-type dump truck. Moreover, the invention isapplicable to not only a dump truck but also a construction vehicleprovided with wheels such as those of a wheel loader.

In the above exemplary embodiment, the vehicle speed estimator 80 isused for estimating the vehicle speed provided by the TCS control.However, the invention may be used for estimating the vehicle speedprovided by an ABS control of all-wheel-drive construction vehicles.

Specific structures and configurations of the invention may be alteredin use in any manner as long as an object of the invention is achieved.

1. A traction control apparatus for an all-wheel drive constructionvehicle, comprising: a vehicle speed estimator that estimates a vehiclespeed, the vehicle speed estimator comprising a rotation speed detectorthat detects a rotation speed of each of wheels and a referencewheel-speed calculator that calculates a reference wheel-speed based onthe rotation speed detected by the rotation speed detector; adriving-force controller that performs a driving-force control of theconstruction vehicle based on the vehicle speed estimated by the vehiclespeed estimator, the driving-force controller comprising a brakingmechanism controller that calculates a slip ratio of each of the wheelsand controls a brake mechanism of the construction vehicle so that thecalculated slip ratio converges to a predetermined target value; avehicle state determiner that determines whether or not the vehiclespeed of the construction vehicle estimated by the vehicle speedestimator and the driving-force control by the driving-force controllerare balanced based on the slip ratio calculated by the braking mechanismcontroller; and a driving-force control changer that changes adriving-force control by the driving-force controller when the vehiclestate determiner determines that the vehicle speed and the driving-forcecontrol are unbalanced.
 2. The traction control apparatus according toclaim 1, wherein the driving-force control changer changes thedriving-force control by setting the vehicle speed estimated by thevehicle speed estimator at the reference wheel speed calculated by thereference wheel-speed calculator.
 3. The traction control apparatusaccording to claim 1, wherein the driving-force control changer changesthe driving-force control by releasing the control by the driving-forcecontroller.